Toxoplasma gondii is the organism that causes toxoplasmosis. In immunosuppressed patients, especially patients with AIDS, toxoplasma encephalitis is a major cause of morbidity and mortality. Although therapies are available for T gondii infection, those therapies currently are associated with a high incidence of adverse reactions. Therefore, new methods to prevent the disease are needed. The use of polynucleotides (naked DNA) encoding target antigens specific for the pathogen offers an exciting new technique to establish immunity. Intradermal or intramuscular injections of plasmid DNA have been shown to provide long-lasting cellular and humoral responses to antigens from a number of infectious agents. These agents include hepatitis B, influenza, HIV, rabies, herpes, malaria, and tuberculosis. In influenza and herpes, significant protection was achieved upon challenge. We have made a plasmid construct in which the P30 surface antigen of T. gondii is expressed under the control of the immediate early promoter of cytomegalovirus. Injection of this plasmid into BALB/c mice through intradermal or intramuscular routes led to a production of antibodies as demonstrated through both enzyme-linked immunosorbent assay and Western techniques. Injection of a control plasmid that contained the gene coding for bacterial b-galactosidase in place of the P30 gene did not elicit a humoral response against the surface antigen. Co-injection of a plasmid expressing granulocyte-macrophage colony-stimulating factor did not significantly enhance the titer of antibody to P30.